Energy from workouts ?

I estimate that I can push an average of 100 watts of sustained leg power to the rear wheel of my bicycle, for about an hour. A guy like Lance Armstrong could maybe sustain 300 watts for four hours, but that's still not much power in the scheme of things.

I added up the 1-year amount of electric and gas energy my house uses for heating, cooling, cooking, lighting, etc., and calculated the average power: It's 30,000 watts! That's the average, over the whole year. It's as if I have 30 people working to keep my house going. And I'm pretty frugal with energy.

Actually, that comparison to people should acknowledge that only about 25% of the food energy I eat can get converted to mechanical energy. So, as comparisons go, it's not necessarily a good one.

On the other hand, the 2,000 calories of food energy I eat each day is the same as about 100 watts of average metabolic power. So if 100 watts is taken as 1 'personpower,' then I, living in this house, am equivalent, in terms of energy consumption to

30 people, not counting myself.

Actually, if I calculate my share of the US energy budget, it works out to about 120 human beings. But, in terms of world averages, I'm equivalent to only about 20 people.

Bob

Quick sanity check here. 30kW is equivalent to 102,000 BTU/hr, which is roughly equivalent to 1 gallon/hr of fuel oil, or 8800 gallons/yr. At current prices that would be around $20,000/yr for energy for you home. Perhaps you slipped a decimal place?

Ned Simmons

I think he must have, unless he has 27 kids and/or a pot greenhouse in his backyard. The easy way to evaluate this problem is this:

Energy burned in a workout = 500 calories (that's about a 3.5 mile run for a 200lb person) There's 4,187 joules per calorie (nutritional), so you basically produce 2 million Joules of energy in a workout (assuming you're 100% efficient, and can harness all your work output). That's the same as .56 kilowatt hours.

That is less than a dime at current electric costs.

As a footnote, the average human is between 30% and 50% efficient in converting energy released via metabolism (i.e. Krebs cycle/electron transport chain) into mechanical energy, depending on the activity and other factors. That alone cuts the .56 kWh in half.

Dave

I expect he meant 30 watts continuous.

But the most efficient recip engines use 0.4 lb fuel per HP hour. so 1 gallon = 6 lb = 15HP hours and 1 gal/hour = 15HP or less thats 15 X 746 watts = 11 kW or less.

Brian Whatcott Altus OK

30 watts? A person uses about 3 times that much sitting. I think 3kW is more likely. 3kW * 24hrs/day * 30 days/month = 2,160 kWh. That's about a $216 electric bill at $0.10 per kWh.

The 1gal/hr assumes 100% efficiency in converting heat to useable energy. 1 gallon of gas has about 131,000,000 J of energy in it. Thats about 37kWh (close to the above calculation of 1 gal/hr). Assuming a

40% efficient engine (30% is good for most car engines), you'd actually need 2.5 gal/hr.

Regardless, 11kW is still really high. I think 3kW is more likely.

Dave

Regardless of the energy produced by the people working out, the biggest problem that I can see with the whole idea of turning workout energy into useful energy is the equipment life costs and the energy produced.

If the cost for purchasing and maintaining some equipment, over its life, is $2,000, but over that same lifetime the energy recovered was equal to $1500, it isn't worth it.

Another problem, as it has been shown here is the efficiency of turning the output energy from a workout into electrical energy. I would like to reiterate that there is no process that will take mechanical energy from a workout and give an equal amount of electrical energy.

So far, so good. This does sound reasonable. (A little high even)

This is getting a little off track. A person spends money to heat the house. Using a gas-fired hot air system is about 80% efficient Using an electrical heating system is 100% efficient.

And I'm not sure if you are arguing for two or three times as much (heating?) fuel as he suggests, or a quarter as much, or one tenth as much. Like to run it up the post again? :-)

Brian Whatcott

Yes, heating the house is efficient. Providing electrical energy and cooling for the house (the majority of your energy use, unless you live in Alaska) is not as efficient.

Well, in retrospect, fuel is really a tangent issue, and it probably doesn't add anything to the discussion. It was used as an index of sorts by Ned, and it doesn't quantify anything we don't already know. However, I was suggesting that more fuel than 1 gal/hr was needed to supply 30kW of power (more like 2.5 gal/hr).

Dave

Dear Brian Whatcott:

...

... neglecting the 40+% efficiency in converting fuel into electricity, and the 1 - 5% loss in delivering the electricity, and the costs associated with operating the "corporation" necessary to generate, maintain, bill you (of course), and manage all those workers. Pound for pound (currency) gas-fired is less expensive.

Of course you cannot use exercise bicycles to fabricate natural gas, but I seem to recall I made quite a bit on my own... ;>)

David A. Smith

Ooops! I mean 3,000 watts continuous. (Yes, I slipped a decimal point.) I appreciate this feedback - and I'm embarrassed by my error. Thanks for correcting me.

But my main point still stands: Human exercise isn't going to contribute much in the way of useful energy - especially after you amortize the cost of making the generators to capture the energy.

One other thing, and it's just a disagreement: I feel like

25% is a closer estimate of the efficiency of how much mechanical work can be got from a human body - unless the exercise is extreme, as in, say, the case of a guy like Lance Armstrong in an uphill sprint.

Bob

It's really dependant on exactly what you're doing. Are you using primarily fast twitch fibers? Slow twitch? How fast are your muscles contracting? (that determines the rate that the myosin and actin cross bridges are connecting and breaking).

Your muscles are actually A LOT more efficient in reverse (i.e. when you lengthening it), because you don't have to put any energy into breaking the cross bridges (the "load" is doing it for you). Cycling and rowing are only about 18% to 27%, similar to your 25%. But the motions of cycling might not be exactly the most efficient, despite the fact that the bike itself IS very efficient. Some studies have quoted over 50% muscle contraction efficiency, such as this study of a thigh muscle contraction (anaerobic vs. aerobic):

Dave

Dave,

Efficiency in this (bicycling) example seems difficult to quantify. What do you think might be a reasonable range to assume for Lance Armstrong in full up-hill sprint?

30% - 40% maybe?

And what do you think a guy like Armstrong might sustain in mechanical output - i.e., to the pedals and rear wheel - for a period of 5 minutes or so? I'd estimate 800 watts.

Bob

Whew, good question... we can take a shot at it:

In order to sustain it, Lance would have to be operating in the aerobic range. Assuming he's drawing about half his energy from carbs and half from fat, for every liter of O2 used, he'd burn about 4.3 calories. (under high intensity, anaerobic conditions, your metabolism switches primarily to carbohydrates, which is less efficient metabolically but can provide large amounts of power). A typical marathone runner can have a VO2 max around 80ml of O2/min/kg, so let's say Lance has about the same stamina.

So if he weighs 165lbs (75kg), and let's say he's operating at 95% of his VO2 max, then he can use 5.7 liters of O2 per minute (95% * 80 *

75) ... That also means he's burning about 24.5 calories/min. Since a calorie is 4,187J, that means he's converting 102.6kJ of energy per minute, or 1.71kW.

The amount of energy from that 1.71kW that is delivered to the wheels is lower. On the low side (25% efficiency), he'd be making 427W. On the high side (say 50%), he'd be delievering 855W.

Of course, some of the numbers used in this calculation are ballparks, but I'm confident they aren't too far off.

Dave

Sanity check #1:

Rule of thumb for runners is 90-110 kCal/mi. burned. A marathoner does a mile somewhere around five minutes -> 24.5 calories/min * 5 min/mi = 122.5 kCal/mi.

Close enough.

Sanity check #2:

The required human power is formidable. Only top competitive cyclists can manage the more than 400 watts necessary to cover 90 km in one hour (0.54 horsepower).

Close enough (pretty damn good!) at the lower efficiency estimates.

Hth, Fred Klingener

Thanks for the feedback.

I was checking into my electricity bill and found the following:

My average monthly comsuption has been of 316.20 KWh, during the past 5 months. Small apartment, no air conditioning, no heating (I live in the tropics at

10 degrees North). Even so it's low.

That's is 316.2 KWh = 316200 Wh / (30 day x 24 h/day) = 440 W approximately. Less than five lamp bulbs of 100 W.

I think there´s something wrong with the meter and the company is undercharging me. Hope they are not reading ;-)

Anyway something in the range of 800 w to 1000 W for my case is more realistic.

OTH, I have always been an advocate of "friendly technology".

I remember watching a TV program some years ago, about a so called Project Gaviota (Seagull) in our neighbours, Colombian countryside, in which there was this fatty guy pedaling in an stationary bicycle, grinding cassava or any other vegetable which needed grinding (maize, etc.).

I was thinking on the first place of such a thing, although I mentioned something about converting and storaging the energy produced in workouts in my original post and I agree with those who pinted out the cost of the equipmente needed for converting and storaging the energy.

I was thinking in doing useful work instead of just working to overcome the artificial friction of the workout equipment or just lifting weights.

Grinding a grain or product that does not need a continuous proccess, for instance. That kind of thing.

I have to think of other processes or uses of that kind of energy.

Regards,

Juan Vazquez.

That's probably about right. Around here, electricity is about $=2E07/kWh, so that would mean a bill of about $22. If you're not using heat or air conditioning, that's probably not too far off. I have found that maintaining the house temperature makes up the lion's share of the electric bill around here (Kansas City, which gets over 100 F and below 0 F). I have had bills as low as $11 in the spring and fall when I have the windows open.

Besides heat and air conditioning, what electricity do you use? The only things that run all the time are clocks, the water heater and the refrigerator (minimal energy). You use lights only when you're at home and then only when a room is dark. You only use cooking appliances at dinner time and washing appliances maybe once a week. Even if you watch several hours of TV a night (shame on you), it doesn't take much more power than a lightbulb.

For a 3000 sq. ft. house with four kids in the Midwest using heat and air and lots of hot water, the figures would be more in the low thousands of kWh.

Don Kansas City

Okay, then 800 watts seems reasonable for an uphill bike sprint lasting a minute, or maybe even several minutes - which I guess, since it's not really sustained all that long, might not be in the aerobic range.

It would be interesting to see, or derive (or otherwise have in hand), a curve of human work output as a function of how long it can be sustained. Years ago (more like decades ago) I could run my 70-kg body up 3 meters (height) of stairs in two seconds (with a running start). That's ~1,000 watts. I'd think that for a burst like that, the efficiency would be pretty high, like >50%, since one's heart and lungs wouldn't even begin to kick into high gear in such a short time.

If you have any thoughts on that, I'd like to hear them.

Fred Klingener (also in response to this) cites 400 watts of output for 1 hour. He ties that with ~25 cal/min (~1,740 watts metabolic) which gives an efficiency, for a

1-hr period, of ~25%, which seems consistent with other data sources.

Bob

PS - This topic of maximum human power output relates to something I've been trying to write: a comparison of individual human dietary energy to per capita cultural energy, as from fossil fuels. The numbers are interesting, but I'm having trouble figuring out how to write it in a way that laypeople can easily grasp. I might start a new thread on it, and see what kind of feedback I get . . .

b

approximately.

That's probably about right. Around here, electricity is about $.07/kWh, so that would mean a bill of about $22. If you're not using heat or air conditioning, that's probably not too far off. I have found that maintaining the house temperature makes up the lion's share of the electric bill around here (Kansas City, which gets over 100 F and below 0 F). I have had bills as low as $11 in the spring and fall when I have the windows open.

Besides heat and air conditioning, what electricity do you use? The only things that run all the time are clocks, the water heater and the refrigerator (minimal energy). You use lights only when you're at home and then only when a room is dark. You only use cooking appliances at dinner time and washing appliances maybe once a week. Even if you watch several hours of TV a night (shame on you), it doesn't take much more power than a lightbulb.

For a 3000 sq. ft. house with four kids in the Midwest using heat and air and lots of hot water, the figures would be more in the low thousands of kWh.

Don Kansas City

My fuel bills are split, I heat water, and run central heating of the house ( small house in England ) by gas, using about 13,500 kWhours annually. My cooking and domestic use, plus some heating of a work shop and running power tools in it use 4,760 kWhours annually. These seem in line with the 316.2 monthly ( 3794 annual ) use quoted above